1. Field of the Invention
The present invention generally relates to a semiconductor plasma CVD apparatus and more particularly to a circuit for measuring DC bias voltage of an ungrounded electrode.
2. Description of the Related Art
Conventionally, plasma CVD apparatuses for forming a thin film on a semiconductor substrate have been widely used. In these plasma CVD apparatuses, the susceptor side is used as one side of electrodes for plasma discharge by grounding it, and the showerhead side is used as the other side of the electrodes for plasma discharge by not grounding it. DC bias voltage (hereinafter referred to as “Vdc”) occurs in an ungrounded electrode (e.g., a showerhead) to which radio-frequency power is applied. This Vdc value is normally measured by a Vdc measurement circuit incorporated in an RF matching circuit.
If an apparatus configuration or process conditions are identical, nearly the same Vdc value is indicated. Consequently, by confirming change in a Vdc value, a difference between apparatuses or apparatus abnormality can be known. Additionally, when a dielectric breakdown occurs in an oxide film on anode and the like coated on an electrode during plasma processing or when abnormal plasma discharge occurs, a Vdc value also fluctuates. Consequently, by monitoring a Vdc value during plasma processing and stopping the plasma processing when abnormal plasma discharge occurs, it becomes possible to prevent damage to a semiconductor substrate or a drop in throughput from occurring. Thus, Vdc value measurement plays an important role in the plasma processing. However, conventionally, the importance of a Vdc value has not been recognized in the art, and problems described below have not been recognized in the art.
Generally, a Vdc measurement circuit comprises several known resistances and a condenser and calculates a Vdc value from resistance values and voltage applied to the resistances.